1. Field of Invention
This invention relates generally to anchor bolt assemblies adapted to fasten fixtures and other objects to masonry, and more particularly to a fail-safe anchor bolt assembly which when installed in a pre-drilled hole in masonry will remain anchored therein even when a crack later develops in the masonry to enlarge the hole.
2. Status of Prior Art
It is often necessary to fasten fixtures and other heavy objects to the surface of brickwork, concrete and other forms of masonry. For this purpose, a conventional practice is to make use of an anchor bolt assembly having an expansible shell which serves to securely retain the bolt within the masonry hole. Anchor bolt assemblies come in diverse forms, but in all such assemblies, some means are provided to bring about the expansion of an expansible shell or a similar component against the wall of the masonry hole to anchor the assembly therein.
Thus in the Giannuzzi U.S. Pat. No. 3,766,819, a bolt whose threaded front end protrudes out of the masonry hole has a waist of reduced diameter adjacent its rear end, the waist being encircled by an expansible shell. The rear end of the bolt has a conical formation such that when during a fastening operation the bolt is slowly withdrawn from the hole by a nut turning on its threaded front end, the rear end cone of the bolt is axially advanced toward and into the shell to expand the shell against the wall of the hole, thereby anchoring the bolt in the masonry.
The primary concern of the present invention is with anchor bolt assemblies of the type disclosed in the Dempsey U.S. Pat. No. 2,988,950 in which a wedge nut is received on the threaded rear end of a headed bolt. When the bolt is turned by a wrench engaging the head, this nut is caused to travel upwardly on the bolt to enter and expand an expansible shell, forcing this shell into tight engagement with the wall of the hole in which the bolt is inserted. One deficiency of bolt anchors of this known type is that under severe vibratory conditions, the bolt may be loosened from the nut, thereby weakening the anchor.
The prior Giannuzzi U.S. Pat. No. 4,195,547 discloses a vibration-proof anchor bolt assembly which is insertable in a hole drilled in masonry for fastening a fixture or other object to the surface thereof, the diameter of the hole being slightly larger than that of the bolt. The bolt is provided with an enlarged head that lies against the fixture and is engageable by a torque tool, the threaded rear section of the bolt being of reduced diameter. The upper portion of the rear section is encircled by an expansible shell which when expanded is forced against the wall of the hole to anchor the bolt therein. Received on the lower portion of the rear section is a wedge nut having an upper conical zone, and a lower cylindrical ring zone whose normal diameter is larger than that of the hole, slots being cut into said ring zone to define spring fingers. These fingers are inwardly flexed by the wall of the hole when the assembly is inserted therein, thereby compressing the ring and constricting the internally-threaded bore thereof.
When the Giannuzzi '547 bolt assembly is fully inserted in the hole and the bolt head is turned by the tool, the wedge nut, whose rotation is arrested by the compressed ring, is axially advanced toward the shell, the conical upper zone of the nut entering the space between the shell and the threaded rear section of the bolt and acting progressively to expand the shell against the wall of the hole until a point is reached where the threaded end of the rear section enters the constricted bore of the compressed ring. The resultant advance of the wedge nut wedges the threaded portion of the bolt between the compressed spring fingers, forcing these fingers outwardly against the hole of the wall, to create pressure between the mating threads which prevents loosening of the bolt under vibratory conditions.
When an expansion-type anchor bolt assembly of the prior art type disclosed above is installed in overhead or side wall masonry, the object fastened to the masonry imposes a load on the anchor bolt which seeks to pull it out of the hole drilled in the masonry. It is vital therefore that in such installations the anchor remain effective under anticipated load conditions. Should the anchor fail and the object become unfastened from the masonry wall, this could endanger human life or have serious economic consequences.
The holding power of an expansion-type masonry anchor of a given size is determined on the basis of a pre-drilled hole of a specified diameter, for when the expansible shell of this anchor is expanded against the wall of the hole by a wedge nut, it is the magnitude of the anchoring force applied by the shell to the wall that is a measure of the holding power of the anchor.
The available holding power depends on the maximum diameter of the wedge nut and the thickness of the shell expanded by the wedge nut. If therefore the outer diameter of the shell is about equal to the diameter of the drilled hole and the maximum diameter of the wedge nut is equal to the outer diameter of the shell, then it is the thickness of the shell that determines the degree to which the outer diameter of the shell can be expanded.
The holding power of an expansion-type anchor is calculated on the basis of a pre-drilled hole in masonry of a specified diameter, it is being assumed that this diameter has a fixed dimension. However, under actual conditions masonry cannot be trusted to remain free of cracks that alter the dimensions of a hole drilled therein. It is known that in concrete, after an anchor is installed therein, that cracks may form in the concrete as a result of vibration and other forces to which the concrete is subjected. The formation of these cracks is exacerbated by the lateral forces produced by an expanded anchor installed in a hole drilled in the concrete.
Should a crack in masonry slice through the wall of a hole drilled therein, the hole wall is then divided into separated sections which effectively enlarge the hole. If the cracked hole has an expansion anchor installed therein whose shell is expanded against the wall of the hole, the force of the expanded shell seeks to widen the separation therebetween, thereby further enlarging the drilled hole and loosening the installation. When a hole is enlarged by a masonry crack, the holding force applied by the expanded shell of the anchor to the wall of the hole is weakened, and the bearing load on the bolt of the anchor produced by the object fastened to the concrete may be great enough to pull the anchor entirely out of the enlarged hole.
Many building officials in the United States and Europe are now concerned that masonry anchors installed in pre-drilled holes be capable of supporting the anticipated load even when a crack develops in the masonry. As indicated by the UEAtc "Technical Guide on Anchors for Use in Cracked and Non-Cracked Concrete" (June 1992) published by the British Board of Agrement, one answer to the problem created by this requirement is to provide undercut anchors which are anchored mainly by a mechanical interlock provided by an undercut in the concrete.
Undercutting is achieved after drilling the cylindrical hole in the masonry by using a special drill before installing the anchor. This undercutting procedure is a time consuming and costly operation, particularly when hundreds of anchors have to be installed at a given site. Moreover anchors of the type adapted for installation in such undercut masonry are more expensive than masonry anchors designed for installation in drilled cylindrical holes.
The need exists therefore for an inexpensive anchor bolt assembly for masonry that is quickly installable in a pre-drilled cylindrical hole and is capable of maintaining an object fastened to masonry even when as result of a crack in this masonry the hole is then enlarged.